Implantable cardioverter-defibrillators, commonly known as ICDs, are cardiac stimulation devices that detect and treat various cardiac arrhythmias including bradycardia (too slow of a heart rate), tachycardia (too fast of a heart rate) and fibrillation. Usually implanted in the pectoral region, an ICD is typically coupled to a patient's heart through transvenous leads that are used to sense electrical signals from the heart and deliver both low voltage and high voltage electrical therapy to the heart to correct detected rhythm dysfunctions. A serious form of cardiac arrhythmia is ventricular fibrillation, which is often fatal if not treated shortly after it begins.
Electrophysiological (“EP”) testing is a procedure that is commonly used to evaluate an individual's susceptibility to cardiac arrhythmias, particularly atrial and ventricular tachycardia. Electrophysiological testing is normally required during an ICD implant procedure and sometimes during follow-up clinical visits. During an electrophysiological study, the ability to induce and terminate tachycardia and/or fibrillation is tested by applying a sequence of high rate pulses, sometimes called “bursts,” that are known to either induce or terminate an arrhythmia. By inducing tachycardia in a patient's heart, a determination can be made as to whether an implanted ICD is functioning properly in detecting and terminating the tachycardia and if its parameters have been optimally programmed.
Electrophysiological testing may include induction of fibrillation and delivery of shocks to determine the defibrillation threshold (“DFT”) for the particular patient. This determination allows the physician to program the defibrillation shocks with an appropriate energy and safety margin. The physician is also able to determine if the selected ICD and implanted electrode configuration will be effective. The implanted electrode configuration may result in unacceptably high defibrillation threshold or the ICD may not have enough energy output to provide a safety margin (typically about 10 joules) over the determined defibrillation threshold. Electrophysiological testing also provides useful diagnostic information to be used by the physician in selecting the most effective tachycardia detection and cardioversion algorithms to be used by the ICD.
Electrophysiological testing, which can also be used to determine if a particular patient would respond favorably to other therapies such as drug therapies used to treat cardiac arrhythmias, has traditionally been an invasive procedure, requiring surgery to place electrodes in contact with desired locations in the patient's heart. Electrical leads couple the implanted electrodes with an external stimulation device capable of delivering bursts of stimulation for the induction of tachycardia as well as low-voltage and high-voltage therapies used to terminate tachycardia.
During an ICD implant procedure, electrophysiological testing can be performed using an external device for delivering electrical pulses to the patients heart by connecting electrical leads to the implanted leads positioned in the heart. A series of carefully timed, high-rate pulses or bursts can be delivered in order to overdrive pace the heart and to induce tachycardia or fibrillation. The external device can deliver cardioversion shocks or high voltage defibrillation shocks, substantially similar to those from an ICD, in order to terminate the induced arrhythmia. In this way, the physician can perform an electrophysiological evaluation of the patient and select the optimal values for parameters to be programmed in the implanted ICD.
However, in order to ease the implant procedure, it is desirable to have the testing functions of an external device incorporated directly into the ICD to eliminate the need for additional equipment. Furthermore, during follow-up office visits, non-invasive EP testing is strongly preferred in order to avoid risks commonly associated with any surgical procedure, such as bleeding, thrombosis or infection.
Non-invasive electrophysiological testing can be performed on patients who have received an implantable cardiac stimulating device such as a pacemaker or ICD. The implanted cardiac stimulation device can be used in a ‘trigger’ mode to track burst stimulation pulses delivered externally by an external stimulation device. The implanted cardiac stimulation device may also be used in an “inhibit” mode in which release of stimulation pulses is controlled by an external stimulation device and a programmer having telemetric communication with the implanted device.
Alternatively, a software or algorithm that is specifically designed for electrophysiological testing may be included in a pacemaker or ICD programmer, eliminating the need for a separate external stimulation device. The ICD programmer is used to send commands to the implanted device to cause administration of burst stimulation in a desired sequence to either induce or terminate an arrhythmia. This process may be referred to as non-invasive programmed stimulation or “NIPS.” For additional details regarding an ICD possessing non-invasive electrophysiological testing capabilities reference is made to U.S. Pat. No. 5,709,711 to Fain, which is incorporated herein by reference.
Certain currently available ICDs have the capability for noninvasive induction and termination of arrhythmias to monitor and test the effectiveness of selected detection criteria and therapies. For example, an ICD may be slaved to an external laboratory stimulator to be used as a timing-signal source to allow signals communicated to the ICD via telemetry to be delivered to the patient with the same timing as the laboratory stimulator output. It also allows the ICD to deliver high-rate pulses through the defibrillation electrodes.
Another ICD provides certain electrophysiological testing capabilities including the ability to confirm the ability to induce the patient's clinical arrhythmia and evaluation of the effectiveness of various ICD therapies in termination of the clinical arrhythmia, when used in connection with an external programmer. While this approach provides certain desirable testing features, it would still be desirable to have full flexibility to allow the ICD to provide all of the functions of an external high voltage stimulator.
One problem with presently available systems is that they provide only limited control over the implanted pulse generator timing and operation. Normally, the pacing operations of the implanted pacemaker or ICD are first disabled in any heart chambers that are not being tested. This can be done by setting the pacing mode to a single-chamber pacing mode or by programming pacing parameters in other chambers to ineffective settings such as a sub-threshold pacing output.
Devices normally operating in a dual-chamber pacing mode are typically set to a single chamber mode during electrophysiological testing in order to prevent high-rate stimulation in one chamber (e.g. the atrium) from causing the other chamber (e.g. the ventricle) to follow the high rate activity. Otherwise, delivery of the high rate stimulation pulses in one chamber may be sensed in another chamber as intrinsic events leading to pacemaker-induced arrhythmia. Thus, pacing support in non-tested chambers is commonly withheld during electrophysiological testing.
Patients having conduction disorders, however, may require pacing support in other chambers during electrophysiological testing. For example, patients lacking atrial-ventricular (AV) conduction require ventricular pacing support during atrial electrophysiological testing. U.S. Pat. No. 5,653,737 to van Lake, which is incorporated herein by reference, describes a dual-chamber pacemaker in which the atrial and ventricular channels can operate simultaneously yet independently so that, during noninvasive electrophysiological testing for atrial tachycardias, the device can generate bursts of pacing pulses to induce and terminate atrial arrhythmias, while maintaining ventricular pacing support. In the '737 patent, the pacing mode is advantageously changed at the start of non-invasive programmed stimulation to an atrial pacing mode which allows burst stimulation for EP testing in the atrium and to an independent ventricular pacing mode which may be VOO, VVT, or preferably VVI, to allow pacing support in the ventricle.
A command from an external stimulator is conventionally used to initiate non-invasive programmed stimulation. Upon receipt of the external command, the pacing mode of the implanted device is changed to the pacing mode desired during the electrophysiological test, often a single-chamber triggered mode. The delivery of the first stimulation pulse of the non-invasive programmed stimulation sequence is triggered only by the external command, thus it is delivered asynchronously with the cardiac cycle.
It would therefore be desirable to include in an implantable cardiac stimulation device, capable of performing non-invasive electrophysiological testing, a method for safely and automatically transitioning from the standard operating mode to a non-invasive programmed stimulation mode in a way that is synchronized to the cardiac cycle. It is further desirable that the delivery of non-invasive programmed stimulation in one cardiac chamber is decoupled from stimulation and sensing occurring in other cardiac chambers. It is still further desirable to provide safe back-up stimulation support in ventricular heart chambers during atrial non-invasive programmed stimulation so that cardiac output can be maintained during the atrial testing. Cardiac synchronized transition back to the standard operating mode of the cardiac stimulation device is desirable at the end of the non-invasive programmed stimulation. Cardiac synchronized transition between non-invasive programmed stimulation and the standard stimulation mode improves device safety by avoiding undesirable asynchronous stimulation of the heart chambers.